1. Field of the Invention
The present invention relates to a multi-stage sprocket assembly for a bicycle.
2. Description of the Related Art
A conventional multi-stage sprocket assembly includes more than two sprockets which have a same tooth thickness predetermined by a drive chain employed in combination with the assembly and also have a same teeth shape. For shifting the drive chain from a smaller-diameter sprocket to a larger-diameter sprocket, the shifting movement of the drive chain occurs only at certain teeth of the larger and smaller sprockets having a predetermined positional relationship relative to each other. This means that the smaller sprocket has a certain number of teeth which satisfy this positional relationship. In the course of the above shifting action of the chain from the smaller sprocket to the larger sprocket, an inner plate portion of the chain remains at a tooth of the smaller sprocket until completion of the chain shift, i.e. until complete disengagement of the chain from the smaller sprocket. Whereas, the shifting chain segment first engages a tooth of the larger sprocket at its outer plate portion subsequent to the above-described inner plate portion. To facilitate understanding of the essential spirits of the present invention, it is defined herein the above inner plate portion as a disengagement-interfering inner plate portion while the outer plate portion is defined as a engagement-leading outer plate portion. Similarly, the tooth of the smaller sprocket engaging with the trailing inner plate portion is defined herein as a disengagement-interfering tooth while the tooth of the larger sprocket engaging with the leading outer plate portion is defined as an engagement-leading tooth, respectively. Further, the disengagement-interfering tooth and the engagement-leading tooth are predetermined according to the numbers of the smaller and larger sprockets, assembled condition between these sprockets and so on.
In the above-described chain shift from the smaller sprocket to the larger sprocket, the shifting chain segement flexes between the inner and outer plates, thus the segment is inclined relative to the rotational direction of the sprocket assembly. Then, for avoiding excessive interference between the shifting chain segment and the sprockets and resultant chain jam problem, the prior art has suggested provision of a convex portion adjacent the engagement-leading tooth of the larger sprocket as a means for smoothing the chain shift. However, the prior art has paid no consideration to the smaller sprocket.
As a result, the inclination angle of the shifting chain segment is determined rather large by the disengagement-interfering inner plate portion and the vicinity of the disengagement-interfering tooth. Consequently, because of the large inclination angle, the chain segment flexes to an excessive degree thus still interfering with the chain shift.
Accordingly, the primary object of the present invention is to provide a multi-stage sprocket assembly for a bicycle with an improvement which can minimize the flexing amount of the shifting chain segment and interference between the disengagement-interfering inner plate portion and the disengagement-interfering tooth thus consequently smoothing the chain shift motion for a change-speed operation.